Anyone conveying letter-post items for others on a commercial basis with an individual weight not exceeding 1,000 g is required to have a licence (section5(1) PostG).
Anyone who is employed by the licensed operator only as a subcontractor is not required to have their own licence (section 5(2) para 1 PostG).
If a letter is merely enclosed in a parcel and the letter relates solely to the content of the parcel (invoice, accompanying letter), then a licence for conveyance is likewise not required (section 5(2) para 2 PostG).
In addition, anyone who conveys letters as a courier is not required to have a licence.
A courier is anyone who delivers individual documented letters directly from the sender to the addressee and has access to the individual item at all times in the event of short-notice instructions from the sender or addressee (section 5(2) para 3 PostG).